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© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 6/9/09

AN-6861

Applying FAN6861 to a Flyback Power Supply with Peak

Load Current Profile

1. Introduction

Highly integrated PWM controller, FAN6861, is optimized

for applications with motor load, such as printers and

scanners, that inherently impose some kind of overload

condition on the power supply during acceleration mode.

The two-level OCP function allows the SMPS to stably

deliver peak power during the motor acceleration without

causing premature shutdown, while protecting the SMPS

from overload condition.

The green-mode and burst-mode functions with a low

operating current (2.2mA maximum in green mode)

maximize the light-load efficiency so that the power supply

can meet stringent standby power regulations.

The frequency-hopping function reduces electro-magnetic

interference (EMI) of a power supply by spreading the

energy over a wider frequency range. The constant power

limit function minimizes the component stress in abnormal

condition and helps to optimize the power stage. Protection

functions; such as OCP, OLP, OVP, and OTP are fully

integrated into FAN6861, which improves the SMPS

reliability without increasing system cost.

This application note presents design considerations to apply

FAN6861 to a flyback power supply with peak load current

profile. It covers designing the transformer, selecting the

components, and closing the feedback loop. Figure 1 shows

a typical application circuit using FAN6861.

L

N

EMI

Filter

+

+

+

AC input

RT

FB

VDD

GATE

SENSE

GND

FAN6861

6

5

4

3

2

1

DD 2

+

KA431

Figure 1. Typical Application